In oil field and other high pressure applications, tubular pipes are connected to one another by pipe couplers or connectors where the connectors utilize a metal-to-metal contact seal for preventing fluid leakage from the interior of the coupling. A metal-to-metal seal in a coupling is principally dependent on the preloading of a sealing member in the coupling to get a tight seal and to prevent leakage when an internal pipe or coupler pressure induces longitudinal stress in the coupling members.
Metal-to-metal seals currently in use are:
1) the API flange--this is a widely used coupling for high pressure fluids and involves a face-to-face coupling of flanges with an interposed metal sealing member between the flanges. The metal sealing member is usually a flat faced seal that is crushed between the two flat faced hubs on the flanges,
2) a "Grayloc" type connector--this connector is comprised of facing metal seal ring hubs and an annular clamp assembly. A internal metal sealing ring resembling an inverted "T" in cross section is disposed between the hubs. The clamp assembly fits over the outer annular surface of the two facing hubs and is forced radially inward by making up the clamp bolts to draw the facing hubs together and to compress with the sealing ring rib between the hub surfaces. As the hubs are drawn together by the clamp assembly, the internal seal ring lips engage and deflect against the inner sealing surfaces on the hubs. The deflection of the seal ring lips elastically preloads the lips of the seal ring against the inner sealing surfaces of the hubs thereby forming a preloaded seal. In use, internal pressure acts on the seal ring lips so that the sealing action of the lips is both preloaded and pressure-energized. However, if the internal pressure becomes sufficiently great to cause the facing hubs to be displaced or moved longitudinally of one another, the seal ring lips will lose their sealing ability because the preloading sealing compression between the hubs and the seal is lost. Also the metal seal ring has a 20.degree. (sometimes 25.degree.) bevel so that the seal lips do not radially compensate for movement of the seal lips when the hubs are spread apart from one another.
3) A "Weco" wing union--this coupler has metal-to-metal compression seals disposed between connector surfaces and uses a lip type elastomer replaceable seal to protect the metal primary seal.
4) A "dynetor" connector--this is a metal-to-metal connector coupling with a reusable annular metal seal. The annular metal seal has cylindrical ends so that some longitudinal expansion or spreading between the coupling parts can occur without losing the seal in the annular seal bores.
5) The Nicholson flange--this is an annular shaped gasket which is used between flanges and flange grooves. The flange seal, when compressed between flanges, will expand with longitudinal movement but the seal will fail because the gasket expands longitudinally and thus will fail at the gap.
6) The Nicholson lip C seal--this is a seal which depends upon point contact for sealing and is a round seal.
7) The Cameron AX or CX gasket illustrated in the 1988-89 Composite Catalog, pgs 683, 685 and 826. The AX and CX gasket is a tubular member which has an external taper on each end and sealing means which engage tapered surfaces in adjoining connectors.